Safari 5.0.4 - google page occasionally unresponsive

Using google, I search on a keyword, visit the first link, press back to return to the search results and click the next hit, go back and so forth. After two or three of these cycles suddenly the google page cannot be navigated anymore. Meaning buttons do not work, links are not clickable and the cursor cannot be placed in the textbox. Only option is to open a new tab and close the current one.

This problem arose recently, but unfortunately some changes to the machine came together i.e. update to Safari 5.0.4, the Java update and the installation of a new logic board (nvidia problem, runs great now). Some time ago I installed firebug, but worked with that for some time without problems.

I did empty cache and repair permissions, problem persists. Does anyone have any suggestions?